Process for the production of sequences of interface layers

Details Process for the production of sequences of interface layers Process for the production of sequences of interface layers

1999-10-19

2002-05-28

Hoffman, John (Department: 1731)

Glass manufacturing

Processes

With destruction or delamination of transitory attached or...

C065S054000, C065S038000

Reexamination Certificate

active

06393868

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a novel process for the production of sequences of interference layers. The invented process permits the production of interference layer systems of varying buildup, which can be used in a large variety of applications. These interference layers systems are suited, in particular, as filters, as interference pigments or as particles to be embedded in documents to prevent counterfeiting.
2. Description of the Related Art
Interference layer systems are composed of any desired number of layers having at least two different refractive indices and layer thicknesses, which are usually smaller than the respective light wavelengths. The interference layers are employed particularly as antireflex layers, reflection layers, interference pigments, beam splitters, edge filters, line filters and minus filters.
There are a number of processes known for producing interference layer systems. These processes are usually coating processes. Thus, chemical processes, such as for instance sol-gel coating processes, spraying processes, surface-reduction processes or CVD (chemical vapor deposition), as well as physical processes, such as for instance vaporization processes or sputtering, can be employed to produce interference layers. A large palette of high-refractive-index and low-refractive-index materials is available as coating and substrate materials.
Interference pigments for mother-of-pearl or metal glaze in lacquers, paints or cosmetics are predominantly produced by coating small platelike mica crystals with TiO
2
or other metal oxides (cf. e.g. U.S. Pat. Nos. 3,553,001 or 3,331,699). Carbon-containing layers and basic organic dyes are also applied as coatings (cf. e.g. EP-0634458 or DE 4225357). Small mica plates with a diameter of 100 to 500 &mgr;m and a thickness of 0.1 to 10 &mgr;m painstakingly gained from natural mica, with a yield of less than 10%, are used as substrates. Other materials are also employed as substrates for interference coatings, such as for example PBSO
4
, small hexagonal Fe
2
0
3
plates (diameter 5 to 50 &mgr;m) and graphite. U.S. Pat. No. 5,436,077 also describes the use of small glass particles as a substrate on which a metal layer and a covering protective layer are deposited. However, mica-based mother-of-pearl and metal glaze pigments are most common and have more than 80% of the market.
The state of the art also describes-coating organic substrates with TiO
2
or ZrO
2
(cf. ZA-6 805 748) or coating preformed polyester-resin-based organic parts of molds (cf. EP-742 262) in order to produce interference pigments. However, the thermal and mechanic stability of these interference pigments generally do not meet the needed requirements.
A drawback of all the known processes and coating materials is the unsatisfactory precision in obtaining the refractive index and thickness of the layers so that the desired interference effect is often not achieved. Especially the refractive index is met only very inadequately due to the employed coating methods. Moreover, the production of sequences of interference layers is very complicated and requires many different process steps, each of which can harbor errors and increase the expense of the entire process. Furthermore, with these processes and materials, a large amount of the layer-forming materials is lost.
Moreover, the layers of the interference layer systems made with these conventional production processes cannot be obtained completely poreless so that the spectral properties may change as water vapor and other gases collect in the pores of the filter. Upon warming up, the gas charge of this interference layer system changes again so that the spectral properties, such as reflection and transmission, vary. In the most unfavorable conditions, e.g. due to environmental influences, the applied coating may also detach from the substrate.
U.S. Pat. No. 3,711,176 describes another process for the production of sequences of interference layers. In this process, high-reflective-index colored plastic films are made from multiple transparent, thermoplastic plastic layers by means of simultaneous extrusion. However, the resulting layer system is usually only suited for obtaining special optical effects on the surfaces due to the inadequate precision in attaining the refractive indices and the layer thicknesses. Moreover, the variation range of the refractive indices of the thermoplastics is only small, which greatly limits the ability to produce a selected interference effect. Moreover, the employed materials are not very thermally, mechanically, chemically or environmentally, e.g. UV radiation, resistant.
The object of the present invention is to provide a process for the production of a sequence of interference layers which permits producing precise and durable interference layer systems at low costs. Furthermore, to provide a sequence of interference layers producible thereby.
BRIEF SUMMARY OF THE INVENTION
The object is solved by providing a process for the production of one of interference filters, interference pigments or interference particles having sequences of interference layers composed of layers i of prescribed thicknesses d(i) and refractive indices n(i), comprising the steps of providing a stack of glass plates having plane surfaces, the stack comprised of at least two layers i of glasses having refractive indices n(i) and thicknesses d
0
(i), which are each larger than the prescribed thicknesses d(i) by the same multiplying factor; heating the stack to a temperature above the transformation temperature of the glasses of the layers; drawing the stack during or after heating in such a manner that the respective layers obtain the prescribed thicknesses d(i); and cooling of the drawn stack.
The object is solved by means of the process according to claim
1
and the sequence of interference layers according to claim
18
. Advantageous embodiments of the process are the subject matter of claims
2
to
16
. An intermediate product, which is obtained in one embodiment of the process is set forth in claim
17
.
The invented process permits precise and inexpensive production of sequences of interference layers of multiple layers i of prescribed thicknesses d(i) and refractive indices n(i), for example a layer sequence of layers 1 (i=1) and 2 (i=2) of varying thicknesses d(1), d(2) and varying refractive indices n(1) and n(2).
An element of the present invention is that it was understood that the production of such a type of sequence of interference layers cannot be realized using conventional coating methods or materials but rather in a very advantageous manner by using glass as the layer material in conjunction with the following process steps.
In this process, first a stack of at least two glass layers i composed of different glasses respectively types of glass, with the same refractive indices n(i) and the same layer sequence as in the to-be-produced sequence of interference layers, are provided. The thicknesses d
0
(i) of the layers of the stack are selected in such a manner that they are always larger by the same factor than the prescribed thicknesses d(i) of the layers of the to-be-produced interference sequence. These parameters can be met in an excellent manner, in particular, with glass materials. The stack is finally heated to a temperature above the transformation temperature of the glasses of the layers and subsequently or simultaneously drawn in such a manner that the individual layers reach the prescribed thicknesses d(i). After this the drawn stack is cooled.
Strikingly, when drawn, the individual glass layers do not melt together in such a manner that they lose their original properties. But rather, both the refractive indices and the layer thicknesses ratio are exactly retained even in the case of large surface layers. The layers melt together only where they touch thereby creating a very advantageous strong bond between the individual precisely defined layers.
In a preferred embodiment of the invented pr

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